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United States Patent |
5,231,985
|
Sutton
,   et al.
|
August 3, 1993
|
Dual chamber rate responsive pacer
Abstract
A cardiac pacemaker and related pacing method. The cardiac pacemaker
includes atrial and ventricular sense amplifiers for generating atrial and
ventricular sense signals. An activity control circuit measures the
activity level and initiates an activity interval. A control circuit
responds to the atrial sense signals, the ventricular sense signals, and
the activity control circuit, for controlling the atrial and ventricular
stimuli generation, by matching the activity interval with the
depolarization of the atrial tissue, in order to differentiate true
exercise induced sinus tachycardia from atrial arrhythmias and retrograde
atrial events, and to permit a selective ventricular rate control. The
control circuit initiates a 2 to 1 ventricular to atrial response when the
activity interval is greater than a VV interval, which is the sum of the
interval between the last sensed or paced ventricular event and the atrial
intrinsic depolarization (VA.sub.S interval) and the programmed AV delay
or the sensed AV interval. The 2 to 1 block mode is induced by prolonging
the post ventricular atrial refractory period (PVARP) for the next beat.
Inventors:
|
Sutton; Richard (London, GB);
Bourgeois; Ivan (Vervier, BE);
Herpers; Loek (Kerkrade, NL);
Dulk; Karl D. (Maastricht, NL)
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Assignee:
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Medtronic, Inc. (Minneapolis, MN)
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Appl. No.:
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648241 |
Filed:
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January 31, 1991 |
Current U.S. Class: |
607/18; 607/123 |
Intern'l Class: |
A61N 001/368 |
Field of Search: |
128/419 PG
|
References Cited
U.S. Patent Documents
4429697 | Feb., 1984 | Nappholz et al. | 128/419.
|
4467810 | Aug., 1984 | Vollmann | 128/419.
|
4554921 | Nov., 1985 | Boute et al. | 128/419.
|
4712555 | Dec., 1987 | Thornander et al. | 128/419.
|
4712556 | Dec., 1987 | Baker, Jr. | 128/419.
|
4722341 | Feb., 1988 | Hedberg et al. | 128/419.
|
4856523 | Aug., 1989 | Sholder et al. | 128/419.
|
4945909 | Aug., 1990 | Fearnot et al. | 128/419.
|
5007422 | Apr., 1991 | Pless et al. | 128/419.
|
5085215 | Feb., 1992 | Nappholz et al. | 128/419.
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5086774 | Feb., 1992 | Duncan | 128/419.
|
Other References
Rate-Responsive Dual-Chamber Pacing by Kappenberger et al., published in
New Perspectives in Cardiac Pacing, edited by S. Serge Barold, MD et al.,
Futura Publishing Co., 1988.
Rate Responsive Dual Chamber Pacing, by Kappenberger et al., published in
PACE, vol. 9, Nov.-Dec. 1986, Part II.
|
Primary Examiner: Howell; Kyle L.
Assistant Examiner: Schaetzle; Kennedy J.
Attorney, Agent or Firm: Kassatly; S. A., Patton; Harold R.
Claims
What is claimed is:
1. A cardiac pacemaker, comprising:
atrial sensing means for sensing depolarizations of the atrium;
ventricular sensing means for sensing depolarizations of the ventricle;
ventricular pulse generating means for generating ventricular pacing
pulses;
A-V delay timer means for timing an A-V interval initiated in response to
sensing of an atrial depolarization by said atrial sensing means and for
triggering generation of an atrial synchronized ventricular pacing pulse
by said ventricular pulse generating means on expiration of said A-V
interval, said atrial synchronized ventricular pacing pulses thereby being
generated separated by intervals corresponding to intervals separating
said sensed atrial depolarizations;
V-V interval timing means for timing a V-V interval following generation of
a ventricular pacing pulse by said ventricular pulse generating means and
for triggering said ventricular pulse generating means to generate a
ventricular pacing pulse at the expiration of said V-V interval;
sensor means for sensing metabolic demand and for defining a variable
sensor interval corresponding to a sensor indicated pacing rate in
response said sensed metabolic demand and for varying the duration of said
V-V interval in correspondence with said sensor interval; and
refractory interval means for timing a variable refractory period following
an atrial synchronized ventricular pacing pulse, for preventing an atrial
depolarization occurring therein from causing said pacemaker to initiate
timing of a said A-V interval, said refractory interval means comprising
means for comparing the interval separating two said atrial
depolarizations prior to delivery of an atrial synchronized ventricular
pacing pulse and said sensor interval in effect prior to delivery of said
ventricular pacing pulse and for extending the duration of said refractory
interval following each ventricular pacing pulse prior to which said
interval separating said atrial depolarizations is less than V-V interval
in correspondence with said sensor interval by at least a predetermined
amount.
2. A pacemaker according to claim 1 further comprising atrial pulse
generating means for generating atrial pacing pulses and wherein said V-V
interval timing means comprises means for timing a V-A interval following
generation of a ventricular pacing pulse by said ventricular pulse
generating means and for triggering said atrial pulse generating means to
generate an atrial pacing pulse at the expiration of said V-A interval and
means for initiating timing of said A-V interval in response to generation
of a said atrial pacing pulse.
3. A cardiac pacemaker according to claim 2, wherein said sensor comprises
means for varying said V-A interval in response to said sensed metabolic
demand, whereby said V-V interval is also varied in response to said
sensed metabolic demand.
4. A pacemaker according to claim 3 wherein said V-A interval equals said
sensor interval.
5. A pacemaker according to claim 4 wherein said means for comparing the
interval separating said two atrial depolarizations prior to delivery of
an atrial synchronized ventricular pacing pulse and said sensor interval
in effect prior to delivery of said atrial synchronized ventricular pacing
pulse comprises means for comparing the interval between the delivery of
said atrial synchronized ventricular pacing pulse and a preceding atrial
synchronized ventricular pacing pulse to said sensor interval.
6. A pacemaker according to claim 3 or 4 or 5 wherein said comparing means
comprises means responsive to the expiration of a said A-V interval prior
to expiration of a said V-A interval, and wherein in response to said A-V
interval expiring prior to said V-A interval, said refractory interval
timing means extends the duration of said atrial refractory interval
following the expiration of said A-V interval.
7. A cardiac pacemaker, comprising:
atrial sensing means for sensing depolarizations of the atrium;
ventricular sensing means for sensing depolarizations of the ventricle;
ventricular pulse generating means for generating ventricular pacing
pulses;
AV delay timer means for timing an AV interval initiated in response to
sensing of atrial depolarization by said atrial sensing means and for
triggering generation of an atrial synchronized ventricular pacing pulse
by said ventricular pulse generating means on expiration of said AV
interval, said atrial synchronized ventricular pacing pulses thereby being
generated separated by intervals corresponding to the rate of said sensed
atrial depolarizations;
V-V interval timing means for timing a V-V interval following generation of
a ventricular pacing pulse by said ventricular pulse generating means and
following sensing of a ventricular depolarization by said ventricular
sensing means and for triggering said ventricular pulse generating means
to generate a ventricular pacing pulse at the expiration of said V-V
interval;
sensor means for sensing metabolic demand and for defining a variable
sensor interval corresponding to a sensor indicated pacing rate in
response thereto and for varying the duration of said V-V interval in
correspondence with said sensor interval; and
synchronization control means for defining a variable maximum
synchronization interval for 1:1 generation of atrial synchronized
ventricular pacing pulses in response to atrial depolarizations, and for
providing 2:1 generation of atrial synchronized ventricular pacing pulses
in response to atrial depolarizations occurring more closely spaced than
said maximum synchronization interval, said control means comprising means
for comparing the rate of two said atrial depolarizations immediately
preceding delivery of an atrial synchronized ventricular pacing pulse and
said sensor indicated pacing rate in effect immediately prior to delivery
of said atrial synchronized ventricular pacing pulse and for defining an
increased maximum synchronization interval in effect for one V-V interval,
following each atrial synchronized ventricular pacing pulse prior to which
said rate of said atrial depolarizations is greater than V-V interval in
correspondence with said sensor interval indicated pacing rate by at least
a predetermined amount.
8. A pacemaker according to claim 7 further comprising atrial pulse
generating means for generating atrial pacing pulses and wherein said V-V
interval timing means comprises means for timing a V-A interval following
generation of a ventricular pacing pulse by said ventricular pulse
generating means and for triggering said atrial pulse generating means to
generate an atrial pacing pulse at the expiration of said V-A interval and
for initiating timing of said A-V interval in response to generation of a
said atrial pacing pulse.
9. A cardiac pacemaker according to claim 8, wherein said sensor comprises
means for varying said V-A interval in response to said sensed metabolic
demand, whereby said V-V interval is also varied in response to said
sensed metabolic demand.
10. A pacemaker according to claim 9 wherein said V-A interval equals said
sensor interval.
11. A pacemaker according to claim 10 wherein said means for comparing the
interval separating two said atrial depolarizations prior to delivery of
an atrial synchronized ventricular pacing pulse and said sensor interval
in effect prior to delivery of said atrial synchronized ventricular pacing
pulse comprises means for comparing the interval between the delivery of
said atrial synchronized ventricular pacing pulse and a preceding
ventricular atrial synchronized pacing pulse to said sensor interval.
12. A pacemaker according to claim 9 or 10 or 11 wherein said comparing
means comprises means responsive to the expiration of a said A-V interval
prior to expiration of a said V-A interval, and wherein in response to
said A-V interval expiring prior to said V-A interval, said
synchronization control means defines said increased maximum
synchronization interval.
13. A pacemaker according to claim 9 or 10 or 11 wherein said
synchronization control means comprises means for defining a variable
refractory period following an atrial synchronized ventricular pacing
pulse, for preventing an atrial depolarization occurring therein from
causing said pacemaker to initiate timing of a said A-V interval, and
wherein said maximum synchronization interval corresponds to the sum of
said A-V interval and said variable refractory interval.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to cardiac pacers, and more
particularly to a dual chamber rate responsive pacemaker which uses
activity sensing to differentiate true exercise induced sinus tachycardia
from atrial arrhythmias and retrograde atrial events, and to permit a
selective ventricular rate control.
2. Background Art
Early pacemakers were asynchronous (VOO), and they stimulated the heart at
a fixed rate, independent of the patient's underlying cardiac rhythm or
metabolic demand. Although such pacers, typified by U.S. Pat. No.
3,057,356 to Greatbatch, provide a ventricular pacing rate sufficient to
sustain life, this pacing mode can compete with native ventricular rhythms
which is undesirable.
Subsequently, demand pacemakers (VVI) were developed. This type of pacer
interacts with the patient's heart to provide stimulating pulses only if
spontaneous cardiac activity does not occur. An example of such a pacer is
taught by U.S. Pat. No. 3,478,746 to Greatbatch. This form of pacer
provides a ventricular sense amplifier for detecting ventricular
depolarizations. A ventricular sense event resynchronizes the pacer's V-V
timer by resetting and restarting it. The ventricular sense event also
cancels or inhibits the scheduled ventricular stimulus and thus avoids
competition with the native ventricular rhythm.
Atrial synchronized pacers (VAT) were developed almost simultaneously with
VVI demand pacemakers. This type of pacer paces the ventricle in response
to the detected atrial rate of the patient. The VAT pacer, as typified by
U.S. Pat. No. 3,253,596 to Keller, provides an atrial sense amplifier for
detecting atrial depolarizations. An atrial sense event starts the pacer's
A-V delay timer. When the A-V timer times out, a ventricular stimulus is
provided. Conceptually, such a pacer can be considered as a prosthetic
conduction pathway which simulates the natural A-V conduction pathways of
the heart. One drawback to this form of pacing is its ability to compete
with ectopic ventricular activity. An ectopic ventricular beat (PVC) may
be detected in the atrium which will result in the generation of a
ventricular stimulus a short time after the ventricular depolarization.
Although such a pacing regime is considered harmless if the A-V delay is
short, it is possible to deliver the pacing stimulus into the vulnerable
period of the ventricle if a premature ventricular contraction occurs, and
thereby initiate an arrhythmia.
Continued development of pacemakers was marked by the invention of the AV
sequential pacer (DVI), as disclosed in U.S. Pat. No. 3,595,242 issued to
Berkovits. This form of pacer provides for stimulation in both the atria
and the ventricles while providing sensing in the ventricle. In this DVI
mode pacer, a ventricular sense event starts both a V-A escape interval,
the pacer delivers an atrial stimulus, and at the end of the V-V escape
interval, the pacer delivers a ventricular stimulus. If a ventricular
sense event occurs during the V-A or V-V time intervals, the pacer will
resynchronize to the ventricular sense event and inhibit the delivery of
the otherwise scheduled ventricular stimulus.
The DDI mode pacer described by U.S. Pat. No. 3,747,604 to Berkovits
further includes an atrial sense amplifier to inhibit the atrial stimulus
if an atrial sense event occurs during the V-A interval. The atrial sense
event does not resynchronize the pacer which makes this device especially
suitable in patients where atrial competition must be avoided.
The atrial synchronized ventricular inhibited or VDD mode pacer, as
disclosed in U.S. Pat. No. 3,648,707 issued to Greatbatch has structures
for sensing in the atrium and ventricle but provides stimulating pulses
only in the ventricle. In operation, the VDD pacer will synchronize on
detected atrial activity and provide a ventricular stimulus if one does
not occur within the A-V delay initiated by the atrial depolarization.
Ventricular sense events inhibit the delivery of the otherwise scheduled
stimulus and resynchronize the pacer's V-V timer.
The dual sense--dual pace DDD mode pacers, have been described in U.S. Pat.
No. 4,312,355 to Funke. The DDD pacer addresses many of the shortcomings
of the prior art devices. The DDD mode pacer, as described by Funke, has
had wide applications. This type of pacer has sense amplifiers for
detecting both atrial and ventricular events, as well as output pulse
circuits for stimulating both the atrium and the ventricle.
This form of prior art pacer provides timing circuitry to initiate an A-V
delay upon the occurrence of an atrial event. If, during the A-V delay
period, no spontaneous ventricular event is sensed, the pacer will produce
a ventricular stimulus at the conclusion of the A-V delay period. If,
during the V-A interval, no spontaneous atrial event is sensed, the pacer
will produce an atrial stimulus at the conclusion of the V-A interval.
In this type of pacemaker, in the absence of spontaneous P waves and R
waves, the heart will be stimulated at fixed AA and VV intervals with a
fixed AV delay. However, if the ventricle depolarizes spontaneously, then
the A-V is truncated and the observed A-A interval is not fixed and will
be shorter than the arithmetic sum of the programmed A-V and V-A
intervals.
The dual chamber modalities, DVI, VAT, VDD and DDD, have proven to be
especially efficacious pacemakers since they restore A-V synchrony and
thus improve cardiac output by accommodating the hemodynamic contribution
of the atrial chambers within the pacing regime. The latter three modes
also synchronize the pacing rate to the patient's native atrial or sinus
rate and thus provide an increased pacing rate in response to bodily
activity. Increasing cardiac rate is the major contributor to increased
cardiac output.
More recently, other pacers which increase cardiac output in response to
exercise have been proposed. They include pacemakers which rely upon the
sensing of a historical average of atrial activity, blood pH, respiratory
rate or QT interval data to alter the pacemaker's escape interval. A
discussion of these background proposals may be found in "The Exercise
Responsive Cardiac Pacemaker", IEEE Transactions on Biomedical
Engineering, Vol. 12, December 1984.
One approach which is important to the understanding of the present
invention is the activity responsive pacer described in U.S. Pat. No.
4,428,378, issued to Anderson et al, and which is incorporated by
reference. The pacer disclosed in that patent monitors the physical
activity of the patient and increases the pacing rate in response to
increasing patient activity.
Other publications that provide background information for the operation of
the present invention include U.S. Pat. No. 4,890,617 issued to Markowitz
et al. which is incorporated herein by reference. This patent describes a
dual chamber activity responsive pacemaker which senses and paces in both
the atrium and the ventricle. The pacing rate is determined by the sensed
activity of the patient, the programmed lower rate, and the patient's
atrial or sinus rate.
U.S. Pat. No. 4,932,046, entitled "Dual Chamber Rate Responsive Pacemaker",
assigned to Medtronic, Inc. of Minneapolis, Minn., which is incorporated
herein by reference, describes a dual chamber rate responsive pacemaker.
The pacemaker operates in an atrial synchronized modality when the sensed
atrial rate is within a physiological range, and paces at a
sensor-determined rate when the atrial rate is above or below the
physiological range.
The Goy et al. article "Rate Response Dual Chamber Pacing" in Centro
Editoriale Italiano, 1986, pages 60-65 describe in general terms, the
clinical results of a rate responsive dual chamber pacemaker.
BRIEF SUMMARY OF THE INVENTION
It is one object of the present invention to disclose a pacemaker which
simultaneously offers dual chamber pacing (DDD Mode) and activity
controlled rate response. The pacemaker uses activity-sensing to
differentiate true exercise induced sinus tachycardia from atrial
arrhythmias and retrograde atrial events, and to permit a selective
ventricular rate control.
Briefly, the above and further objects and features of the present
invention are realized by a cardiac pacemaker and a related pacing method.
The cardiac pacemaker includes an atrial sense amplifier for generating
atrial sense signals in response to the depolarization of the atrial
tissue. A ventricular sense amplifier generates ventricular sense signals
in response to the depolarization of the ventricular tissue. An AV delay
timer responds to the atrial sense amplifier for initiating an AV delay
interval. An atrial stimulator generates atrial stimuli, and a ventricular
stimulator generates ventricular stimuli.
An activity control circuit measures the activity level and initiates an
activity interval. A control circuit responds to the atrial sense signals,
the ventricular sense signals, and the activity control circuit, for
controlling the atrial and ventricular stimuli generation by the atrial
and ventricular stimulator, respectively by matching the activity interval
with the depolarization of the atrial tissue, in order to differentiate
true exercise induced sinus tachycardia from atrial arrhythmias and
retrograde atrial events, and to permit a selective ventricular rate
control.
The control circuit initiates a 2 to 1 ventricular to atrial response by
prolonging the post ventricular atrial refractory period when the activity
interval is greater than a VV interval, which is the sum of the interval
between the last sensed or paced ventricular event and the atrial
intrinsic depolarization (VA.sub.S interval) and the programmed AV delay
or the sensed AV interval.
The method for cardiac pacing includes the steps of determining whether a P
wave has been sensed; comparing the activity interval and the VV interval;
and, if the activity interval is greater than the VV interval, then
comparing the interval between the two previous atrial paced or sensed
events, (PP interval) and the pacemaker maximum synchronization interval.
The pacemaker is then caused to pace in a 2 to 1 block mode if the PP
interval is less than the maximum synchronization interval, and to pace in
a 1 to 1 synchronous mode if the PP interval is greater than, or equal to,
the maximum synchronization interval.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects and features of the present invention and the
manner of attaining them, will become apparent, and the invention itself
will be best understood, by reference to the following description and the
accompanying drawings, wherein:
FIG. 1 is a block diagram of a pacemaker according to the present
invention;
FIG. 2 is a software flickered illustrating the general operation logic of
the pacemaker of FIG. 1 according to the present invention;
FIG. 3 is an exemplary rate diagram of the pacemaker of FIG. 1 showing the
variation of the atrial--ventricular rate with time, as a function of
activity; and
FIG. 4 is an exemplary rate diagram of the pacemaker of FIG. 1 showing the
variation of the ventricular pacing change with respect to the atrial
sense.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The general concept of integrating activity sensing and dual chamber pacing
in a pacemaker is published and generally described in "Rate Responsive
Dual Chamber Pacing", PACE Journal, Vol. 9, November-December 1986, Part
II, pages 987-991, by Lukas J. Kappenberger and Loek Herpers.
The authors of this article recognize that the use of pacemakers which
reestablish or maintain atrioventricular synchrony is limited, due to the
fact that many patients with AV conduction disturbances have additional
sinus disease. Event though the use of a single chamber rate-responsive
atrial pacemaker is possible in patients with sick sinus syndrome, sick
sinus syndrome patients have, or may develop, additional AV-node disease,
and require the use of a dual chamber rate responsive pacemaker.
Additionally, conventional rate responsive pacemakers are not without their
inherent problems, in that rate-responsive ventricular pacing does not
protect against the hemodynamic problems that can arise with the loss of
timed atrial systole. Pacemaker syndrome and palpitations due to
retrograde conduction have not yet been satisfactorily addressed.
FIG. 1 illustrates the main elements of a pacemaker 10 according to the
present invention. The pacemaker 10 can be either external or implantable.
The pacemaker 10 generally includes an atrial sense amplifier 22, an
atrial pulse or output generator 26, a ventricular sense amplifier 24, a
ventricular pulse or output generator 25, a control circuit 16, an
activity control circuit 27 and an activity sensor 30.
With the exception of the interface control circuit 16, the circuit designs
of the pacemaker 10, can be adapted from circuit designs of conventional
rate responsive and DDD pacemakers. The operation of the interface control
circuit 16 will be described hereinafter in greater detail.
Hardware suitable for practicing the present invention includes Medtronic's
Activitrax (R) pulse generators Models 8400, 8402 and 8403, and
Medtronic's Symbios (R) Models 7005 and 7006. The pacemaker 10 is a
programmable computer based unit which interfaces with the heart 20
through the atrial and ventricular sense amplifiers 22 and 24
respectively, as well as through the atrial and ventricular pulse or
output generators 26 and 28 respectively. U.S. Pat. No. 4,577,633, which
is incorporated by reference, describes one possible computer driven
stimulator which can be used to practice the present invention. This
patent is incorporated herein by reference.
The sensor 30 measures the metabolic demand of the patient. A suitable
sensor is disclosed in U.S. Pat. No. 4,428,378 to Anderson and Brumwell,
which sets forth the structure for monitoring the physical activities of
the body to set a pacing rate. This patent is incorporated herein by
reference.
The following Table I summarizes the operation logic of the novel pacemaker
10:
TABLE I
______________________________________
BRIEF SUMMARY OF OPERATION LOGIC
Activity Interval/
Mode of
Event P Wave Sensing
DDD Intervals
Operation
______________________________________
1. No Activity DVIR - DDIR
Interval .ltoreq.
VA Interval
2. No Activity DDD/DDI
Interval > at LR
VA Interval
3. Yes Activity VDD - 1:1
PP Rate .ltoreq. ATUR
Interval .ltoreq.
Synchrony
VV Interval
4. Yes Activity VDD -
PP Rate > ATUR
Interval .ltoreq.
Wenckebach
VV Interval at UR
5. Yes Activity VDD - 1:1
PP Interval .gtoreq. Max.
Interval > or .ltoreq.
Synchrony
Synchronization
VV Interval
Interval
6. Yes Activity VDD - 2:1
PP Interval < Max.
Interval > Block
Synchronization
VV Interval
Interval
______________________________________
The pacemaker 10 can be programmed to operate as a dual chamber demand
pacemaker (DDD) without backup pacing rate control by means of activity
sensing, at a demand responsive rate varying between a programmable lower
rate ranging, for instance, between 50 and 90 pulses per minute (ppm), and
a programmable atrial tracking upper rate (ATUR) ranging, for instance,
between 125 and 175 ppm. When the pacemaker 10 paces at the atrial
tracking upper rate, it operates in Wenckebach mode.
Once the pacemaker 10 is programmed in the DDD and activity modes according
to the present invention, it operates in a substantially similar way to a
conventional DDD pacemaker, with the exception that pacing in both the
atrial and ventricular chambers is continuously regulated by the body
activity detection circuitry within the range defined by the programmed
basic rate (i.e. 60-70 ppm) and the programmed maximum activity rate (i.e.
120-140 ppm).
FIG. 3 illustrates an exemplary rate diagram of the pacemaker 10, and shows
the variation of the atrial--ventricular rate with time, as a function of
activity. The atrial tracking upper rate (ATUR) and the programmed maximum
activity rate can be separately programmable to provide the physician more
flexibility in programming the pacemaker. The physician is able to select
different values for the ATUR and the maximum activity rate.
Atrial synchronized ventricular pacing (VDD) up to the upper rate (UR) will
be obtained if the interval between the last sensed or paced ventricular
event and the atrial intrinsic depolarization (VA.sub.S interval) is
shorter than the interval set by the activity sensor, and if the activity
interval is shorter than the sum of the VA.sub.S interval and the
programmed AV delay or sensed AV interval.
However, if the activity interval is longer than the sum of the VA.sub.S
interval and the programmed AV delay or sensed AV interval, then the
control circuit 16 will cause the atrial and ventricular pulse generators
26 and 28 to switch to an atrio-ventricular response of 2 to 1, such that
a ventricular synchronized stimulus is generated for every other P wave,
when the P rate is above a programmable maximum synchronization rate (i.e.
90 to 120 ppm). If on the other hand, the P rate is below or equal to the
maximum synchronization rate, the atrio-ventricular response is
synchronized (i.e. 1 to 1).
Consequently, the pacemaker 10 shows a unique feature in that it uses
activity sensing to differentiate true exercise induced sinus tachycardia
from atrial arrhythmias and retrograde atrial events, and to permit a
selective ventricular rate control.
It is desirable to cause a rapid P-wave synchronized ventricular pacing to
be possible only in the presence of activity detection. This behavior is
achieved by causing the atrial and ventricular pulse generators 26 and 28
to switch to different synchronization modes, for preventing pacemaker
reentrant tachycardia. As a result, the pacemaker 10 restores most of the
physiologic cardiac rhythm with optimal hemodynamic improvement and thus
optimal benefit to the patient.
The conversion of body activity to rate response is achieved by
transforming the body's internal pressure changes into changes in atrial
and ventricular pacing rates. This is achieved by using a piezoelectric
element similar to the sensor used in the Medtronic Activitrax (R) pulse
generator. These modifications allow continuous control of the backup rate
of DDD pacing through the activity sensor, and therefore permit
rate-responsive dual chamber pacing (DDDR).
The pacemaker 10 can be programmed using a Medtronic 9710 programmer. The
programmable parameters are: mode (VOO, VVI, DOO, DVI, DDD), activity
(yes/no), basic rate, AV interval, activity threshold, rate of response,
atrial refractory period, pulse width A and V, pulse amplitude A and V,
and sensitivity in A and V.
The operation logic of the pacemaker 10, illustrated in Table I, will now
be described in detail, in relation to the flickered 200 of FIG. 2, and
the rate diagram 800 of FIG. 5.
The software program illustrated by the flickered 200 of FIG. 2 is
initiated at 202 by a ventricular event, and an inquiry is made at 204,
whether atrial depolarization has been sensed. If it has not, then a
determination is made at 206 whether the activity interval is shorter
than, or equal to, the pulse generator programmed VA interval. A
ventricular event resets the activity timer and the VA counter. A sensed
or paced atrial event resets the AA counter and the AV counter.
If the activity interval is shorter than, or equal to, the programmed VA
interval, then the pacemaker 10 operates in the DVIR mode (AV sequential
pacing), or DDIR mode as illustrated by block 208, and by Event 1 in Table
I. If on the other hand, the activity interval is longer than the
programmed VA interval, then the pacemaker 10 will pace at the programmed
lower rate (LR) in a DDD mode, as indicated by block 210 and Event 2.
If at block 204 a P-wave is sensed, then the activity interval is compared
to a VV interval. As used herein, the VV interval is defined as the sum of
the VA.sub.S interval and the programmed AV delay, or, alternatively, as
the sum of the VA.sub.S interval and the sensed AV interval.
If the activity interval is less than, or equal to the VV interval, then
the control circuit 16 compares the PP rate to ATUR, as indicated by block
214. As used herein, the PP interval refers to the interval between the
previous atrial paced or sensed event, and the atrial depolarization.
If at 214 it is determined that the PP rate is lower than or is equal to
ATUR, then, as indicated by block 216, 1 to 1 tracking will prevail, until
the PP rate is higher than ATUR block 218), at which time the pacemaker 10
will slow its rate of ventricular pacing by the Wenckebach operation, 2 to
1 block, or another appropriate block. In a DDD pacemaker, the Wenckebach
mode is an operational function which limits the average ventricular
pacing rate when the intrinsic atrial rates rise above the programmed
upper rate. The pacemaker does this by gradually prolonging the
pacemaker's AV interval until one of the atrial events falls into the
atrial refractory period and is not sensed. Since no AV interval is
started, there will be no ventricular output synchronized to this atrial
event.
Table I exemplifies the above findings as Events 3 and 4, and FIG. 4
illustrates these events as lines 802 and 804, respectively.
If at 212 it is determined that the activity interval is greater than the
VV interval, then, as indicated by block 220, the control circuit 16
compares the PP interval to the maximum synchronization interval. As
defined herein, the maximum synchronization interval is defined as the sum
of the AV interval and the post ventricular atrial refractory period
(PVARP). PVARP is the blanking period after a ventricular sensed or paced
event, during which the pacemaker 10 cannot sense an atrial event. The
purpose of this blanking period is to prevent the atrial sensing circuit
from detecting ventricular signals and retrogradely conducted signals.
If the PP interval is greater than, or equal to, the maximum
synchronization interval, then 1 to 1 tracking (block 222) will prevail
(Event 5), until the PP interval is shorter than the maximum
synchronization interval (block 224), at which time the pacemaker 10 will
slow its rate of ventricular pacing by 2 to 1 block (Event 6). This can be
done by prolonging the PVARP for one beat.
Therefore, in these Events 5 and 6, even though atrial depolarization is
sensed prior to the timing out of the activity sensor, the activity timer
is allowed to time out up to a ventricular sensed event or a ventricular
paced event, and the activity interval is compared to the to the VV
interval. If the PP interval is longer than the maximum synchronization
interval, which is defined by the atrial refractory period,
atrio-ventricular synchronous pacing will occur (Event 5).
The PVARP is prolonged for one beat if the activity does not time out at a
ventricular event. The PVARP prolongation defines the maximum
synchronization interval. If an atrial event occurs during the PVARP, it
will not result in ventricular pacing (Event 6).
In Event 6, the activity time out point is considered to be too distant
from the sensed P-wave, such that the sensed atrial depolarization is
treated as an atrial arrythmia or retrograde atrial event, and is ignored.
As a result, the pacemaker 10 uses activity-sensing to differentiate true
exercise induced sinus tachycardia from atrial arrhythmias and retrograde
atrial events, and permits a selective ventricular rate control.
In the preferred embodiment, the 2 to 1 block is induced by prolonging the
PVARP after the ventricular event for one cardiac cycle. For instance, the
PVARP can be of the order of 200 milliseconds, and the prolonged PVARP
period can be of the order of 400 milliseconds. It should however become
apparent to the persons skilled in the art that other PVARP values could
be selected between appropriate ranges to accomplish similar results. The
PVARP could be prolonged from one value to another value in a single step
increment, or it could be prolonged in a series of incremental steps. By
way of an alternative, the PVARP could be modulated as a function of the
rate difference between the sinus rate and the activity rate.
While in the above example the PVARP has been prolonged to accomplish the
desired result, it should be understood that the atrial refractory period
(ARP) could alternatively be prolonged to accomplish similar results.
While particular embodiments of the present invention have been disclosed,
it is to be understood that various different modifications are possible
and are contemplated within the scope of the specification, drawings,
abstract and appended claims.
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